Wikipedia
The Cone of Silence is one of many recurring joke devices from Get Smart, an American comedy television series of the 1960s about an inept spy. The essence of the joke is that the apparatus, designed for secret conversations, makes it impossible for those inside the device - and easy for those outside the device - to hear the conversation. The end result being neither secret nor communication.
In popular culture, "cone of silence" is a slang phrase meaning that the speaker wishes to keep the indicated information secret and that the conversation should not be repeated to anyone not currently present. For example: "We aren't inviting Cindy and her boyfriend to the movies because they embarrass us, but keep that in the cone of silence."
Cone of Silence is a 1960 British drama film directed by Charles Frend and starring Michael Craig, Peter Cushing and Bernard Lee. The film is about the investigation into a series of crashes involving the fictional "Atlas Aviation Phoenix" jetliner. Cone of Silence is loosely based on the 1952 crash in Rome and investigations into the structural integrity of the de Havilland Comet. The film was based on David Beaty's novel, Cone of Silence (1959), later renamed Trouble in the Sky, the title of the film, as released in the United States.
Usage examples of "cone of silence".
They moved out of the cone of silence and, side by side, walked toward the clumps of Houses Minor at the end of the hall.
They showed no cone of silence but, on the other hand, street people were ignored so completely by the rest of the citys residents it amounted to the same thing.
But I might as well have been arguing in a cone of silence, for all that Hani listened to me.
Soon, at the vortex of the wards, she confronted the cone of silence and darkness which had formerly defeated all her skills.
Outside her cone of silence she could see Leo watching her intently.